version 3.1.0.0 but runtime v2.0.50727

Error 2 The type 'Microsoft.Practices.EnterpriseLibrary.Common.Instrumentation.IInstrumentationEventProvider' is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.Practices.EnterpriseLibrary.Common, Version=3.1.0.0,
Culture=neutral, PublicKeyToken=null'.C:\Users\me\Documents\Visual Studio 2005\Projects\uNet\Chart.Log\Logger.cs 393 33 Chart.Log

I have added a reference and am using 'Microsoft.Practices.EnterpriseLibrary.Common.Instrumentation;'. I am guessing the problem is that I only have NET2 installed on this PC and I don't want to install NET3 - because I want to be sure my programs are only
using NET2.

The 'Microsoft.Practices.EnterpriseLibrary.Common reference properties refer to version 3.1.0.0 but runtime v2.0.50727.

Based on your error "...Instrumentation.IInstrumentationEventProvider..." I bet you need to install the instumentation that came with EntLib. There are links to the batch file(s) for installing the instrumentation under start -> programs -> entlib

Then you can add the reference in the usual way. If your not using instrumentation you could take it out of your config.